How to disable a button in Flutter

In iOS, we can easily control a button enabled/disabled state by setting an isEnabled property for UIKit and a disabled(_:) modifier for SwiftUI. In Flutter, you won't find such attributes.

// SwiftUI
Button("Click Me!", action: {})
    .disabled(true)

// UIKit
let button = UIButton(type: .system)
button.isEnabled = false

How to disable a button in Flutter

Flutter pushes declarative UI to its limit in this case. Enable and disable the state of a button resulting from the present of onPressed callback.

If the onPressed callback is null, Flutter treat the button as no action hence showing the button in a disabled state.

If the onPressed is there, Flutter shows the button in an enabled state.

Here is an example of buttons in an enable and a disable state.

ElevatedButton(
  onPressed: () {},
  child: Text('Click Me!'),
),
ElevatedButton(
  onPressed: null,
  child: Text('Click Me!'),
),

The first button contains onPressed callback, so Flutter shows it in an enabled state. Flutter doesn't care about the activities within the callback. It only checks for its existence.

The second button has a null callback, showing as a disabled state.

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How to conditionally change a button enable/disable state

Normally, we would want to enable and disable a button based on some condition. For example, I want a button to enable once a user accepts terms and conditions.

To do that, we need to do two things.

1. Declare a bool variable or function that returns a bool. This will use to control a button state.
2. Use that variable or function to set the onPressed callback conditionally.

In the following example, we enable a button only when a user accepts the terms and conditions checkbox.

class HomePage extends StatefulWidget {
  const HomePage({Key? key}) : super(key: key);

  @override
  _HomePageState createState() => _HomePageState();
}

class _HomePageState extends State<HomePage> {
  // 1
  bool _isAcceptTermsAndConditions = false;

  @override
  Widget build(BuildContext context) {
    return Scaffold(
      appBar: AppBar(
        title: Text('Flutter App'),
      ),
      body: Center(
        child: Column(
          mainAxisAlignment: MainAxisAlignment.center,
          children: [
            Row(
              mainAxisAlignment: MainAxisAlignment.center,
              children: [
                Checkbox(
                    value: _isAcceptTermsAndConditions,
                    onChanged: (value) {
                      setState(() {
                        // 2
                        _isAcceptTermsAndConditions = value ?? false;
                      });
                    }),
                Text('I accept the terms and conditions.'),
              ],
            ),
            ElevatedButton(
              // 3
              onPressed: _isAcceptTermsAndConditions
                  ? () {
                      print('Submit');
                    }
                  : null,
              child: Text('Click Me!'),
            ),
          ],
        ),
      ),
    );
  }
}

1 We declare a bool variable, _isAcceptTermsAndConditions, to keep user choice.
2 When checkbox value change, we change _isAcceptTermsAndConditions value.
3 We use _isAcceptTermsAndConditions value to conditional set onPressed callback.

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Conclusion

A Flutter button enabled and disabled state is controlled by a present of the onPressed callback.

Disabled button

We set the onPressed callback to null to disable a button.

onPressed: null

Enabled button

We set the onPressed callback to a non-null function to enable a button.

onPressed: () { print('action') }

Conditionally enable and disable a button

To control an enable and disable state dynamically, we use a ternary operation (?:) to set the onPressed callback.

var isEnabled = false

onPressed: isEnabled ? () {}: null